An explosion at the headquarters of Mexico's state-owned oil company killed 25 people and injured 101 on Thursday as it heavily damaged three floors of a building, sending hundreds into the streets and a large plume of smoke over Mexico City's skyline.

Rescuers continued to search the rubble for victims trapped in the debris late Thursday with the aid of rescue dogs, trucks with mounted lights and an oil company crane. Interior Minister Miguel Osorio Chong said it was uncertain if there were any people still trapped but that crews would keep searching.

Many of the office workers were outside having lunch when the explosion occurred about 3:45 p.m. local time in a basement parking garage next to the 51-story tower of Petroleos Mexicanos, or Pemex, one of the tallest buildings in Mexico City.

Osorio Chong said the explosion hit the basement and first two floors of the 14-story building.

"It was an explosion, a shock, the lights went out, and suddenly there was a lot of debris," employee Cristian Obele told Milenio television, adding that he had been injured in the leg. "Co-workers helped us get out of the building."

There was no immediate cause given for the blast, which also damaged the first and second floors of the auxiliary building in a busy commercial and residential area. But in a Tweet, Pemex said it had evacuated the building as a precautionary measure because of a problem with the electrical system.

The company later tweeted that experts were analyzing the explosion and that any reports of a cause were speculation.

The tower, where several thousand people work, was evacuated. The main floor and the mezzanine of the auxiliary building, where the explosion occurred, were heavily damaged, along with windows as far as three floors up.

A reporter at the scene saw rescue workers trying to free several workers trapped. Television images showed people being evacuated by office chairs and gurneys. Most of them had injuries probably caused by falling debris.

Police landed four rescue helicopters to remove the dead or injured. About a dozen tow trucks were furiously moving cars to make more landing room for the helicopters.

Streets surrounding the building were closed as evacuees wandered around, and rescue crews loaded the injured into ambulances.

"I profoundly lament the death of our fellow workers at Pemex. My condolences to their families," President Enrique Peña Nieto said via his Twitter account.

Shortly before the explosion, Operations Director Carlos Murrieta reported via Twitter that the company had reduced its accident rate in recent years.